The ability of some materials to generate an electric potential in response to applied mechanical stress is referred to as piezoelectricity. Established piezoelectric materials are lead zirconium titanate (PZT) and fluorinated polymers such as polyvinylidene fluoride (PVDF). Piezoelectric behaviour has also been observed in foamed, closed-pore polypropylene (PP). In order to achieve piezoelectricity, such a polypropylene foam is charged in a high electric field. As a result, electrical breakdowns occur within the pores, generating macrodipoles and polarising the material macroscopically. Such polypropylene ferroelectrets can have a piezoelectric coefficient of several hundred picocoulombs per Newton. In order further to increase the sensitivity of the sensor action, multilayer systems comprising a plurality of foams stacked one above the other have been developed.
Gerhard et al. (2007 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pages 453 to 456) describe a three-layer ferroelectret in which a polytetrafluoroethylene film provided with a plurality of homogeneous through-holes by mechanical or laser-based drilling is arranged between two homogeneous fluoroethylenepropylene films. However, the incorporation of through-holes by mechanical or laser-based drilling is complex and unsuitable for large-scale production. For example, as is also disclosed in the above-mentioned source (p. 454), the perforated layer must be chemically cleaned after the mechanical or laser-based drilling in order to remove metal residues (burrs) or organic residues.
The three-layer ferroelectret disclosed in Gerhard et al. is produced by a lamination process. In this process, the polytetrafluoroethylene film provided with holes is joined to the fluoroethylenepropylene films by passing the layers between two heated rotating cylinders under high pressure and at elevated temperature (310° C.).
It would be desirable to provide a process for the production of an electromechanical, for example piezoelectric, converter that is suitable for large-scale production.